Ground-state phase diagram of ultrasoft bosons

In two-dimensional bosonic systems ultrasoft interactions develop an interesting phenomenology that ultimately leads to the appearance of supersolid phases in free space conditions. While suggested in early theoretical works and despite many further analytical efforts, the appearance of these exotic phases as well as the detailed shape of the ground-state phase diagrams have not been established yet. Here we develop a variational mean-field calculation for a generic quantum system with cluster-forming interactions. We show that by including the restriction of a fixed integer number of particles per cluster the ground-state phase diagram can be obtained in great detail. The great detail includes the determination of coexistence regimes of crystals of different occupancy as well as crystals with superfluid phases. To illustrate the application of the method we consider the softened van der Waals potential, for which the phase diagram is known via quantum Monte Carlo simulations. For densities other than that corresponding to the single-particle crystal, our results show very good quantitative agreement with the simulations regarding the location of the superfluid transitions. Additionally, the phase diagrams suggest that the solid-superfluid coexistence could be a reliable marker to locate supersolidity.

Automated summary

In two-dimensional bosonic systems with ultrasoft interactions, supersolid phases can appear in free space conditions. Through a variational mean-field calculation with cluster-forming interactions, detailed ground-state phase diagrams were obtained, showing coexistence regimes of crystals with different occupancy and superfluid phases. The study suggests that the solid-superfluid coexistence could be a reliable marker to locate supersolidity.